I can't say if this is the kind you are supposed to use but it said "non-toxic" so that is good enough for this nurse.
Basically, you just put a bunch of ink on your kid's foot and stamp it onto the cards. You may need to distract them with chips, blankies and Cars movies but the end result is worth it.
Yeah, kind of a rip off of the Mothers' Day cards but I love it anyway.
FYI, your kid's foot will be black for a few days. This is what Sully's foot looked like after I scrubbed it 3 times.